Encroachment of woody vegetation drives rapid state change in insular grasslands
Rapid ecosystem state changes, such as transitions from grasslands to forest, can have significant consequences for biodiversity and ecosystem function. Factors causing such state changes and appropriate management strategies for maintaining stability, however, remain poorly understood. In this study, we used historic aerial photos to analyze rates of woody encroachment into Ozark glades (fire-adapted grasslands) and potential drivers of these state changes. We hypothesized that local environmental variables, landscape spatial context, and prior woody cover would all affect state changes. We compared long-term change in glades that have been managed with prescribed fire and mechanical thinning in recent decades with long-unmanaged glades.
We found that woody vegetation has increased significantly across the landscape since 1939, with only minor sensitivity to local environmental factors and landscape context. Previous cover of woody vegetation was, however, a stronger predictor of encroachment rates, suggesting that propagule arrival exerts a larger influence on encroachment than environmental filters. Woody vegetation cover was higher in unmanaged glades than managed glades, but contemporary managed glades still have significantly higher woody vegetation cover than they did in 1939. Our findings suggest that woody encroachment in glades exhibits hysteresis, meaning that restoring encroached glade landscapes will likely require more intensive management efforts that maintaining existing open glades.